On the formation of metal-rich deposits at ridge crests

Data from the hot springs at the Galapagos spreading center (T = 3–13°C) show depletions of the exiting waters in Cu, Ni, Cd, Se, Cr and U relative to ambient seawater. Manganese is strongly enriched. Iron shows highly variable behavior between vent fields but is in general low. The data confirm the occurrence of extensive subsurface mixing between the primary high-temperature, acid, reducing hydrothermal fluids and “groundwater”. The composition of the latter is indistinguishable from that of the free water column adjacent to the ridge axis. The final solutions are on the boundary between those forming MnO₂ crusts and those producing iron-manganese rich sediments. The suite of metal rich deposits observed at ridge crests — Mn-O, Fe-Mn-O, Fe-S — can be explained as the manifestation of the degree of subsurface mixing, decreasing from 100 : <1 to <1 : 1 across the series (assuming an end-member temperature of 350°C).     

Crystal morphologies in pillow basalts: implications for mid-ocean ridge processes

We present the results of a detailed petrological study of a sparsely phyric basalt (MAPCO CH98-DR11) dredged along the Mid-Atlantic Ridge (30°41′N). The sample contains microphenocrysts of olivine that display four different rapid-growth morphologies. Comparison of these morphologies with those obtained in dynamic crystallization experiments allows us to constrain the thermal history of the sample. The dendritic morphology (swallowtail, chain and lattice olivine) is directly related to the final quenching during magma–seawater interaction. In contrast, the three other morphologies, namely the complex polyhedral crystal, the closed hopper and the complex swallowtail morphology result from several cycles of cooling–heating (corresponding to a maximum degree of undercooling of 20–25°C) during crystal growth. These thermal variations occurred before eruption and are interpreted to be the result of turbulent convection in a small magmatic body beneath the ridge. The results suggest that the Mid-Atlantic Ridge is underlain by a mush zone that releases batches of liquid during tectonic segregation. Aphyric basalts are emitted during eruptions controlled by the tectonic activity, whereas phyric basalts correspond to small fractions of magma from the mush zone mobilized by reinjections of primitive magmas.     

Nitrogen isotope geochemistry of basaltic glasses: implications for mantle degassing and structure?

The nitrogen isotope geochemistry of 15 basaltic glasses has been investigated using stepped heating and high sensitivity static vacuum mass spectrometry. At low temperature (< 600°C) the glasses release small amounts of nitrogen with δ¹⁵N_AIR, averaging −0.3‰, suggesting surficial adsorption of atmospheric nitrogen. At high temperature, usually with a maximum at 1000°C, indigenous nitrogen with a concentration ranging from 0.2 to 2.1 ppm is released. The δ¹⁵N values of this high temperature release show a wide range from −4.5‰ to +15.5‰. There is no correlation between N ppm and δ¹⁵N, and the samples apparently form 3 groups with distinctive δ¹⁵N. Six MORB glasses from the Mid-Atlantic Ridge, East Pacific Rise and Juan de Fuca Ridge define a group with δ¹⁵N = +7.5 ± 1.3‰. In contrast two Indian Ocean MORB glasses (Carlsberg Ridge and Gulf of Aden) gave negative δ¹⁵N averaging −3.2‰. Glasses from Loihi Seamount have high δ¹⁵N averaging +14.0 ± 1.0‰. Comparison of the δ¹⁵N data with the mantle models derived from helium and argon isotope studies suggests that the wide range in δ¹⁵N may reflect in part heterogeneities in the mantle related to its degassing history. It is possible, however, that magmatic degassing processes have also affected nitrogen isotopic compositions, and the data cannot yet be unambiguously interpreted in terms of source variations.     

Sedimentary rocks at Meridiani Planum: Origin, diagenesis, and implications for life on Mars

The MER rover Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks, exposed in craters and along fissures in Meridiani Planum, are sandstones formed via the erosion and re-deposition of fine grained siliciclastics and evaporites derived from the chemical weathering of olivine basalts by acidic waters. A stratigraphic section more than seven meters thick measured in Endurance crater is dominated by eolian dune and sand sheet facies; the uppermost half meter, however, exhibits festoon cross lamination at a length scale that indicates subaqueous deposition, likely in a playa-like interdune setting. Silicates and sulfate minerals dominate outcrop geochemistry, but hematite and Fe3D3 (another ferric iron phase) make up as much as 11% of the rocks by weight. Jarosite in the outcrop matrix indicates precipitation at low pH. Cements, hematitic concretions, and crystal molds attest to a complex history of early diagenesis, mediated by ambient ground waters. The depositional and early diagenetic paleoenvironment at Meridiani was arid, acidic, and oxidizing, a characterization that places strong constraints on astrobiologial inference.     

Recent pH and CO2 profiles at Lakes Nyos and Monoun, Cameroon: implications for the degassing strategy and its numerical simulation

In situ pH profiles are reported for the first time for Lakes Nyos and Monoun. The pH profiles were converted to CO₂ profiles using HCO₃⁻ profiles calculated from conductivity data. Recent observations (1993–1996) at Lake Nyos indicates that CO₂ still accumulates below 180 m depth at a rate of 125 Mmol year⁻¹. At Lake Monoun, the majority of CO₂ is present below a depth of 60 m, only 25 m below the saturation depth. Consequently, a potential danger of gas explosion is high at both lakes, and artificial degassing of the lakes should be performed as soon as possible. A system for industrial degassing of the lakes is proposed. The system, based on the self-sustained gas lift principle, consists of multiple pipes (14 cm in diameter) with different intake depths; 12 pipes for Lake Nyos (four each at 185, 195 and 205 m) and three pipes for Lake Monoun (at 70, 80 and 90 m). The stepped degassing at different depths is intended to keep the maximum stability of the lakes. The proposed degassing operation was simulated using the code for both lakes. In 5 years, approximately 50% of currently dissolved CO₂ in Lake Nyos and 90% in Lake Monoun will be removed. The expected changes in the thermal and chemical structures of the lakes as degassing proceeds will be most easily monitored with a carefully calibrated CTD equipped with a pH sensor. The simulation indicates that the discharged degassed water will sink to a level of neutral buoyancy, i.e. to a maximum of 70 m at Lake Nyos and 35 m at Lake Monoun. There would be no possibility of triggering a gas explosion by this plunge of discharged water because the water present there would have already been replaced by water at lower CO₂ concentration, during the degassing from shallower pipes.     

Apatite compositions from oceanic cumulates with implications for the evolution of mid-ocean ridge magmatic systems

Apatite distributions and compositions from cumulates from the EPR at Hess Deep, the MAR at the Kane Transform, and the SWIR at ODP Hole 735B were determined to assess the variability at each setting and to evaluate the potential utility of apatite in understanding the evolution of the lower ocean crust. Apatite in cumulates with low P₂O₅ contents are heterogeneously distributed along linear arrays or in tight clusters suggesting they crystallized from planar or pipe-like channels of evolved liquid. Most of the variation in the apatite composition is in the halogen site. The X_Ap^F and X_Ap^Cl are inversely correlated defining trends at near constant X_Ap^OH with EPR X_Ap^OH=0.25, MAR X_Ap^OH=0.45, and SWIR X_Ap^OH=0.65. These trends are defined both by sample averages and by the range of individual analyses in samples with large ranges in their F/Cl ratio. These trends are interpreted to reflect the reequilibration of apatite from variably evolved liquids/fluids that moved through the crystal mush. Comparison of the F–Cl–OH contents of apatite with the F–Cl–OH contents of glasses recovered from the same ridges show approximate correspondence: apatite and glasses from the EPR have the highest halogen contents while apatite and glasses from the MAR and SWIR have more OH. Some of the higher Cl contents in the apatite are interpreted to be produced by degassing of the cumulus pile while others reflect the assimilation of a seawater component. We suggest that systematic analysis of apatite from oceanic cumulates might allow cumulates that crystallized at shallow depths and assimilated seawater-derived components to be distinguished from those that crystallized below the level of seawater interaction.     

The Río Tinto Basin, Spain: Mineralogy, sedimentary geobiology, and implications for interpretation of outcrop rocks at Meridiani Planum, Mars

Exploration by the NASA rover Opportunity has revealed sulfate- and hematite-rich sedimentary rocks exposed in craters and other surface features of Meridiani Planum, Mars. Modern, Holocene, and Plio-Pleistocene deposits of the Río Tinto, southwestern Spain, provide at least a partial environmental analog to Meridiani Planum rocks, facilitating our understanding of Meridiani mineral precipitation and diagenesis, while informing considerations of martian astrobiology. Oxidation, thought to be biologically mediated, of pyritic ore bodies by groundwaters in the source area of the Río Tinto generates headwaters enriched in sulfuric acid and ferric iron. Seasonal evaporation of river water drives precipitation of hydronium jarosite and schwertmannite, while (Mg,Al,Fe³⁺)-copiapite, coquimbite, gypsum, and other sulfate minerals precipitate nearby as efflorescences where locally variable source waters are brought to the surface by capillary action. During the wet season, hydrolysis of sulfate salts results in the precipitation of nanophase goethite. Holocene and Plio-Pleistocene terraces show increasing goethite crystallinity and then replacement of goethite with hematite through time. Hematite in Meridiani spherules also formed during diagenesis, although whether these replaced precursor goethite or precipitated directly from groundwaters is not known. The retention of jarosite and other soluble sulfate salts suggests that water limited the diagenesis of Meridiani rocks.Diverse prokaryotic and eukaryotic microorganisms inhabit acidic and seasonally dry Río Tinto environments. Organic matter does not persist in Río Tinto sediments, but biosignatures imparted to sedimentary rocks as macroscopic textures of coated microbial streamers, surface blisters formed by biogenic gas, and microfossils preserved as casts and molds in iron oxides help to shape strategies for astrobiological investigation of Meridiani outcrops.     

Coupled degassing and crystallization: experimental study at continuous pressure drop, with application to volcanic bombs

 Experiments on degassing of water-saturated granite melts with a pressure drop from 100 and 450 MPa to 40 and 120 MPa, respectively, at temperatures close to feldspar liquidus (750–700  °C), were carried out to determine the modality of water exsolution and vesicle formation at the liquidus temperature. Pressure-drop rates as small as approximately 100 bar/day were used. Uniform space distributions of bubbles of exsolved water were obtained with starting glass containing a small fraction (≈0.5 vol.%) of trapped air bubbles. Volume crystallization of feldspar was observed in degassed melts supplied with seeds. Bubble size distributions (BSD) measured in granite glasses after degassing are presented. Data on vesicle characteristics (number, radius, area, elongation) were acquired on images digitized with standard software, while the reconstruction of size distributions was performed with the Schwartz-Saltikov "unfolding" procedure. Bubble size distributions of size classes in the range 5–1000 μm were acquired with proper magnification and satisfactory statistical reliability of determined number densities. The BSDs of the experimental samples are compared with the results of measurements of rapidly degassed products of Mt. Etna and Vulcano Island. Many particular features of the bubble nucleation and growth can be distinguished in an individual BSD. However, the general BSD of the whole data set, including natural ones, can be relatively well described with linear regression in bilogarithmic coordinates. The slope of this regression is approximately 2.8±0.1. This dependence is in striking contrast with distributions theoretically predicted with classical nucleation models based on homogeneous nucleation of vesicles. The theoretical distribution requires the occurrence of strong maxima that are not observed in our experimental and natural samples, thus arguing for heterogeneous nucleation mechanisms. Received: 1 October 1998 / Accepted: 25 June 1999     

Geochemistry of basalts from the Indian Ocean triple junction: implications for the generation and evolution of Indian Ocean ridge basalts

Basalts dredged from ridge axes within 70 km of the Indian Ocean triple junction in the western Indian Ocean have many geochemical and petrologic characteristics in common with depleted mid-ocean ridge basalts (MORBs) from the Atlantic and Pacific. For example there is overlap in major and trace element abundances, and in diagnostic ratios such as K/Rb (700–925) and La/Sm (less than chondritic). Also, glass inclusions in calcic plagioclase (An_89–90) provide evidence for a primitive high Mg/Fe, low TiO₂ melt. In contrast, basalts dredged from 250 to 400 km southwest of the triple junction on the Southwest Indian Ridge are compositionally distinct from depleted MORB. They are nepheline-normative or slightly hypersthene normative and have higher alkali metal and incompatible element abundances than depleted MORBs with similar MgO contents.All of these Indian Ocean basalts have Sr, Nd and Pb isotope ratios which corroborate previous studies showing that relative to depleted Atlantic and Pacific MORB, many Indian Ocean MORBs have low²⁰⁶Pb/²⁰⁴Pb and high⁸⁷Sr/⁸⁶Sr. However, individual Indian Ocean ridges have different radiogenic isotope characteristics, and basalts from the vicinity of the triple junction have unusually high⁸⁷Sr/⁸⁶Sr (∼ 0.7032) at low²⁰⁶Pb/²⁰⁴Pb ratios (17.3–18.2). Moreover, the shallow axial region of the Central Indian Ridge from ∼ 12°S to the triple junction (26°S) has high⁸⁷Sr/⁸⁶Sr (> 0.7030). Apparently, the depleted component of Indian Ocean MORBs has been contaminated by an isotopically unusual component which does not occur in Pacific and Atlantic MORBs, and is not dominant in basalts from many Indian Ocean islands. The degree of this contamination is not uniform in western Indian Ocean MORB; the most contaminated basalts occur from 12°S on the Central Indian Ridge to the triple junction (∼ 26°S) and easterly along the Southeast Indian Ridge to ∼ 72°E.     

Adakitic rocks at convergent plate boundaries: Compositions and petrogenesis

Science China Earth Sciences - Adakitic rocks are intermediate-acid magmatic rocks characterized by enrichment in light rare-earth elements, depletion in heavy rare-earth elements, positive to...     

Evidence for initial36Ar in volcanic rocks,and some implications

This paper extends the evidence that volcanic rocks may contain initial argon, and investigates some of the implications of the results.The incorporation of initial argon with a composition different to that of air at the time of crystallisation may be an important cause of scatter of K-Ar dates in areas such as the British Tertiary igneous province, but in favourable circumstances the correct age may be recovered.An age of about 60 Ma is indicated for the lavas of Northern Ireland.     

Arc-type and intraplate-type ridge basalts formed at the trench-trench-ridge tripIe junction: Implication for the extensive sub-ridge mantle heterogeneity

Abstract ' In situ basalts' represent the ridge magmatism at and close to the ancient trench-trench-ridge triple junction. Such basalts in the Amami, Mugi, and Setogawa accretionary complexes, Southwest Japan, were described and analysed. The geochemical data show that the ' in situ basalts' include all the types of basalts, ranging from alkali basalts and high-alumina basalts to tholeiites, and the compositions tend towards intermediate and silicic rocks. The data also reveal that the ridge basalts are indistinguishable both from the island arc and intraplate basalts, no affinities with mid-ocean-ridge basalts. The sub-ridge mantle adjacent to the triple junction had a component of sub-arc mantle, and this mantle heterogeneity can be generated by the formation of a slab window.     

Petrographical and geochemical signatures of Jurassic rocks of Chari Formation,Western India:implications for provenance and tectonic setting

The sandstones of the Ridge and Athleta members of Chari Formation(Callovian-Oxfordian)exposed at Jara have been analyzed for their petrographical and geochemical studies. Texturally, these sandstones are medium to coarse grained, poorly to well sorted, sub-angular to sub-rounded, and show low to medium sphericity.These sandstones were derived from a mixed provenance including granites, granite-gneisses, low and high-grade metamorphic, and some basic rocks of Aravalli range and Nagarparkar massif. The petrofacies analysis reveals that these sandstones belong to the continental block and recycled orogen tectonic regime. The studied sandstones are modified by paleoclimate, distance of transport, and diagenesis. Mineralogically and geochemically, sandstones are classified as quartzarenite, subarkose, arkose, sublithic arenite, and wacke, respectively. The A-CN-K ternary plot and CIA, CIW, PIA, and ICV values suggest that the similar source rocks suffered moderate to high chemical weathering under a hot-humid climate in an acidic environment with higher PCO_2. Generally good to strong correlations between Al_2O3 and other oxides in these sediments indicate clay mineral control. The K_2O/Na_2O versus SiO_2 diagram indicates that the studied samples occupy passive margin fields but the SiO_2/Al_2O_3 versus K_2O/Na_2O plot suggests that the Athleta Sandstone and Ridge Sandstone fall within the passive margin field, while Ridge Shale falls within the active continental margin field.     

Bed waves at the basin scale: implications for river management and restoration

An extensive literature about fluvial sediment waves, slugs or pulses has emerged in the past 20 years. The concept has been useful in many respects, but has been applied to diverse phenomena using a variety of definitions. Moreover, inferred linkages between channel‐bed changes and sediment loads are often not justifiable. This paper reviews concepts of large fluvial sediment waves at scales extending to several tens of kilometres. It points out constraints on the inferences that can be made about sediment loads based on changes in channel‐bed elevation at this scale where channel sediment interacts with storage in floodplain and terrace deposits. The type area of G. K. Gilbert's initial sediment‐wave concept is re‐examined to show that neither wave translation nor dispersion occurred in the simple manner commonly assumed. Channel aggradation and return to graded conditions provide an alternative theory explaining Gilbert's observed bed‐elevation changes. Recognizing the evidence and implications of the former passage of a large‐scale bed wave is essential to the accurate diagnosis of catchment conditions and the adoption of appropriate river restoration goals or methods. Sediment loads, water quality, channel morphologic stability and aquatic ecosystems often reflect changes in sediment storage long after the channel bed has returned to grade. Copyright © 2006 John Wiley & Sons, Ltd.     

Ash fallout scenarios at Vesuvius: Numerical simulations and implications for hazard assessment

Volcanic ash fallout subsequent to a possible renewal of the Vesuvius activity represents a serious threat to the highly urbanized area around the volcano. In order to assess the relative hazard we consider three different possible scenarios such as those following Plinian, Sub-Plinian, and violent Strombolian eruptions. Reference eruptions for each scenario are similar to the 79 AD (Pompeii), the 1631 AD (or 472 AD) and the 1944 AD Vesuvius events, respectively. Fallout deposits for the first two scenarios are modeled using HAZMAP, a model based on a semi-analytical solution of the 2D advection–diffusion–sedimentation equation. In contrast, fallout following a violent Strombolian event is modeled by means of FALL3D, a numerical model based on the solution of the full 3D advection–diffusion–sedimentation equation which is valid also within the atmospheric boundary layer. Inputs for models are total erupted mass, eruption column height, bulk grain-size, bulk component distribution, and a statistical set of wind profiles obtained by the NCEP/NCAR re-analysis. We computed ground load probability maps for different ash loadings. In the case of a Sub-Plinian scenario, the most representative tephra loading maps in 16 cardinal directions were also calculated. The probability maps obtained for the different scenarios are aimed to give support to the risk mitigation strategies.     

Measurement of bubble size distributions in vesiculated rocks with implications for quantitative estimation of eruption processes

This paper outlines methods for determining a bubble size distribution (BSD) and the moments of the BSD function in vesiculated clasts produced by volcanic eruptions. It reports the results of applications of the methods to 11 natural samples and discusses the implications for quantitative estimates of eruption processes. The analysis is based on a quantitative morphological (stereological) method for 2-dimensional imaging of cross-sections of samples. One method determines, with some assumptions, the complete shape of the BSD function from the chord lengths cut by bubbles. The other determines the 1st, 2nd and 3rd moments of distribution functions by measurement of the number of bubbles per unit area, the surface area per unit volume, and the volume fraction of bubbles. Comparison of procedures and results of these two distinct methods shows that the latter yields rather more reliable results than the former, though the results coincide in absolute and relative magnitudes. Results of the analysis for vesiculated rocks from eleven subPlinian to Plinian eruptions show some interesting systematic correlations both between moments of the BSD and between a moment and the eruption column height or the SiO₂ content of magma. These correlations are successfully interpreted in terms of the nucleation and growth processes of bubbles in ascending magmas. This suggests that bubble coalescence does not predominate in sub-Plinian to Plinian explosive eruptions. The moment-moment correlations put constraints on the style of the nucleation and growth process of bubbles. The scaling argument suggests that a single nucleation event and subsequent growth with any kind of bubble interaction under continuous depressurization, which leads to an intermediate growth law between the diffusional growth ( ) at a constant depressurization rate and the Ostwald ripening ( ) under a constant pressure, where R_m and t are the mean radius of bubble and the effective time of diffusion respectively, occurred in the eruptions. It is emphasized that the BSD in vesiculated rocks from terrestrial volcanoes can be used to estimate quantitatively eruption processes such as the initial saturation pressure and magma ascent velocity in a volcanic conduit.     

A thermo-plastic constitutive law for brittle-plastic behavior of rocks at high temperatures

Mechanical properties of rocks change under the influence of, temperature. Stress at the onset of yielding, ultimate strength, dilatancy, strain hardening and softening, and the confining pressure at brittle-ductile transition are all reduced by the increasing temperature. This study presents a framework of constitutive modeling of thermo-brittle-plastic behavior of rocks which encompasses these changes. The constitutive law is based on a thermo-plasticity theory first proposed for metals byPrager (1958). Two phenomenological mechanisms have been identified as central for the modeling: temperature dependence of the yield locus (thermal softening), and temperature dependence of the strain-hardening function (thermally enhanced ductility). Material parameters for two rocks, Carrara marble and Westerly granite, were determined on the basis of additional hypotheses. These parameters are used in numerical simulations of triaxial tests at different temperatures. The obtained stress-strain curves compare well to the experimental results. The changes with temperature in the stress at the onset of yielding are more accurately reproduced that the evolution of hardening or softening. Suggestions for possible improvements and future research directions are indicated.     

Oblique subduction, collision of microcontinents and subduction of oceanic ridge: Their implications on the Cretaceous tectonics of Japan

Abstract The Izanagi plate subducted rapidly and obliquely under the accretionary terrane of Japan in the Cretaceous before 85 Ma. A chain of microcontinents collided with it at about 140 Ma. In southwest Japan the major part of it subducted thereafter, but in northeast Japan it accreted and the trench jumped oceanward, resulting in a curved volcanic front. The oblique subduction and the underplated microcon-tinent caused uplifting of high-pressure (high-P) metamorphic rocks and large scale crustal shortening in southwest Japan. The oblique subduction caused left-lateral faulting and ductile shearing in northeast Japan. The arc sliver crossed over the high-temperature (high-T) zone of arc magmatism, resulting in a wide high-T metamorphosed belt. At about 85 Ma, the subduction mode changed from oblique to normal and the tectonic mode changed drastically. Just after this the Kula/Pacific ridge subducted and the subduction rate of the Pacific plate decreased gradually, causing the intrusion of huge amounts of granite magma and the eruption of acidic volcanics from large cauldrons. The oblique subduction of the Pacific plate resumed at 53 Ma and the left-lateral faults were reactivated.     

Paleomagnetism and magnetic fabrics of Mio-Pliocene hypabyssal rocks of the Combia event,Colombia: tectonic implications

Mio-Pliocene hypabyssal rocks of the Combia event in the Amagá basin (NW Andes-Colombia), contain a deformational record of the activity of the Cauca-Romeral fault system, and the interaction of terranes within the Choco and northern Andean blocks. Previous paleomagnetic studies interpreted coherent counterclockwise rotations and noncoherent modes of rotation about horizontal axes for the Combia intrusives. However, rotations were determined from in-situ paleomagnetic directions and the existing data set is small. In order to better understand the deformational features of these rocks, we collected new paleomagnetic, structural, petrographic and magnetic fabric data from well exposed hypabyssal rocks of the Combia event. The magnetizations of these rocks are controlled by a low-coercivity ferromagnetic phase. Samples respond well to alternatingfield demagnetization isolating a magnetization component of moderate coercivity. These rocks do not have ductile deformation features. Anisotropy of magnetic susceptibility and morphotectonic analysis indicate that rotation about horizontal axes is consistently to the south-east, suggesting the need to apply a structural correction to the paleomagnetic data. The relationships between magnetic foliations and host-rock bedding planes indicate tectonic activity initiated before ~10 Ma. We present a mean paleomagnetic direction (declination D = 342.8°, inclination I = 12.1°, 95% confidence interval α₉₅ = 12.5°, precision parameter k = 8.6, number of specimens n = 18) that incorporates structural corrections. The dispersion S = 27° of site means cannot be explained by secular variation alone, but it indicates a counterclockwise rotation of 14.8° ± 12.7° relative to stable South America. Paleomagnetic data within a block bounded by the Sabanalarga and Cascajosa faults forms a more coherent data set (D = 336.5°, I = 17.4°, α₉₅ = 11.7°, k = 12.5, n = 14), which differs from sites west of the Sabanalarga fault and shows a rotation about a vertical axis of 20.2° ± 10.7°. Deformation in the Amagá basin may be tentatively explained by the obduction of the Cañas Gordas terrane over the northwestern margin of the northern Andean block. However, it can also be related to the local effects of the Cauca-Romeral fault system.